A fault current limiter is a device that is installed in an electric power system or the like. An installed fault current limiter can reduce a fault current caused by a short circuit or the like so as to reduce damage on a device connected thereto.
While typical conventional fault current limiters are of the reactor type, superconducting fault current limiters have been proposed in recent years. Such superconducting fault current limiters have a superconducting element inserted in a current pathway, which remains in a superconducting state while a current flows within a defined current range but changes to a normal conducting state in the event of a fault current that exceeds the critical current of the superconducting element so as to reduce the fault current by virtue of its resistance.
The superconducting element used in such fault current limiters is cooled by being immersed in a liquid coolant such as liquid helium and liquid nitrogen in order to maintain an ultralow temperature condition. In the event of a fault current, the superconducting element changes to a normal conducting state, and the element temperature is drastically increased.
A fault current limiter is used generally along with a fault current breaker. In the event of a fault current, it is required to reduce the fault current until the breaker works (approximately 0.1 [s]). Unless the superconducting element is cooled immediately once the breaker cuts off the fault current, the element life is largely decreased, which eventually decrease the life of the fault current limiter.
In particular, when a plurality of superconducting elements are used, a superconducting element with the lowest critical current tends to generate a heat for the longest time and to have a shorter life than the other superconducting elements. Eventually the life of the overall device is decreased to a great extent.
To cope with the problem, superconducting fault current limiters require rapid cooling of the superconducting element when it generates a heat.
For example, Patent Document 1 discloses a cooling structure that includes a coolant vessel that accommodates a semiconductor element as a heat source and a liquid coolant, a partition wall that divides the inner space of the coolant vessel into a coolant supplying area and an element disposed area, a coolant jetting port that is a nozzle extending from the partition wall toward the semiconductor element, and a disturbance forming member that disturbs the flow of the coolant near the outlet of the coolant jetting port.
In this cooling structure, the liquid coolant is jetted from the coolant jetting port toward the semiconductor element by using supplying pressure of the coolant, and the disturbance forming member disturbs the flow of the coolant so that nucleate boiling occurs on the overall flat surface of the semiconductor element as uniformly as possible. With this configuration, high cooling efficiency is achieved.
Patent Document 2 discloses a cooling structure that includes a coolant vessel that accommodates a semiconductor element as a heat source and a liquid coolant, a cooling pipe disposed between the inner and outer walls of the coolant vessel, and a plurality of protrusions that extend from the inner wall of the coolant vessel toward the semiconductor element so as to trap bubbles.
In this cooling structure, the protrusions capture bubbles of the coolant that are produced due to the increased temperature of the semiconductor element, so as to proactively cool down the bubbles. With this, high cooling efficiency is achieved.
Patent Document 3 discloses a cooling device that includes a coolant vessel that houses a semiconductor element and liquid coolant, and a cooling fin disposed on the outer wall of the coolant vessel.
In this cooling structure, the semiconductor element is disposed on the inner bottom of the coolant vessel, and a flow path heading right above from the semiconductor element and a flow path that returns the coolant to the bottom around the semiconductor element are formed. The liquid coolant heated by the semiconductor element rises along the center flow path and is then cooled by the cooling fin when it flows down along the outer flow path. With this circulation, improved cooling efficiency is achieved.
Patent Document 4 discloses providing a coolant vessel that accommodates a liquid coolant and a superconducting device as a component to be cooled, a partition plate that surrounds the superconducting device in the coolant vessel, a heat exchanger that independently cools the inner area and the outer area of the partition plate. The coolant is cooled such that the inner area of the partition plate is cooler than the outer area, so that the coolant flows down in the inner side of the partition plate and flows up in the outer side of the partition plate. With this configuration, improved cooling efficiency is achieved.